A study by consumer champion Which? has found that many existing customers of banks, insurers and energy companies are being charged for calls whilst new customers benefit from free calls.
Existing customers are being charged premium rates to complain to companies who provide them with a service.
Which? surveyed 34 companies and found that 27 of them offered free 0800 numbers for new customers but just six of the 34 allowed existing customers to call the free numbers.
20 of the companies analysed made existing customers call 0845 numbers to complain about poor service.
Churchill Insurers, Scottish Power, HSBC and Natwest are just some of the companies that charge existing customers premium rates to talk about their accounts or make a complaint.
Which? discovered a wide variety of different numbers that customers are expected to use depending on the company and the type of call they are making.
To make a 20-minute call to a 0845 number from a BT landline costs around 50p. This rises to over £1 to call a 0844 or 0871 number. The charge increases still further to over £2 to call an 0871 number. If you use a mobile the costs are even higher, depending on your network.
Which? found that over half of the companies failed to give accurate and clear details over call charge costs on their websites. Those that did included British Gas, Smile, Npower, Santander and Barclays.
Which? executive director Richard Lloyd said: "It is unbelievable that companies would add insult to injury by charging their customers a premium to make a complaint. It's even worse when they offer new customers access to 0800 numbers while leaving loyal customers to pay more.
"We want to see providers being fairer to their existing customers and being more transparent on their call charges so that people are clear what it will cost before they pick up the phone."
A spokesman from Churchill said that calls to 0845 numbers were capped to customers local rates.
The company statement said:"The 0800 numbers are used for new customers looking to take up our insurance cover and 0845 for existing customers seeking changes to policies or making claims. We cap the 0845 numbers to local rate so existing customers can take time to discuss their policy with our experts without worrying about premium rate phone calls."
Scottish Power said that many calls to 0845 numbers are free anyway since BT launched a free calls service to 0845 numbers from landlines in January 2009.
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